The present invention relates to a laminated packaging material comprising a core layer and a layer of cross-linked polyvinyl alcohol applied on one side of the core layer and acting as gas barrier. The present invention further relates to a method of producing the laminated packaging material, as well as to packaging containers produced from the laminated packaging material and possessing superior tightness properties vis-à-vis liquids and gases, in particular oxygen gas.
Within packaging technology, use has long been made of packages of the disposable type (so-called single use packages) for packing and transporting products such as, for example, liquid foods.
A predominant group of these single use disposable packages is produced from a laminated packaging material which comprises a core layer of paper or paperboard and outer, liquid-tight coatings of plastic, preferably polyethylene, on both sides of the core layer.
The point of departure and prime object of the composition of the packaging material is to impart to the package the best possible mechanical and chemical product protection properties for the packed product, at the same time as making possible simple and rational production of the package. A core layer of paper or paperboard imparts to the package the requisite mechanical configurational stability so that the package can be conveniently handled in a rational distribution system, while the outer plastic coatings efficiently protect the liquid-absorbent fibre layer against penetration of moisture and liquid which otherwise would rapidly render the package sloppy and unusable. If the outer liquid-tight plastic coatings consist of polyethylene, the advantage is also afforded that the laminated packaging material is given excellent heat-sealing properties such that a package may be permanently given its desired geometric configuration by conventional heat sealing technology during conversion of the laminated packaging material into finished packages.
A laminated packaging material consisting solely of paper or paperboard and plastic as described above thus makes for a rational production of configurationally stable, liquid-tight packages, but however lacks tightness properties vis-à-vis gases, in particular oxygen gas and must therefore be supplemented with such gas tightness properties in order to be usable for packages for oxygen gas-sensitive products such as, for example, foods.
It is known in the art to supplement the described paper or paperboard material with a foil of aluminum (so-called Alifoil) which serves as a gas barrier and is applied on the one side of the core layer between the core layer and the one outer liquid-tight plastic coating of the laminated packaging material. An intact Alifoil is practically completely gas tight, but because of the slight (or non-existent) extensibility and ductility of the Alifoil, there is a risk that the once applied Alifoil cracks in particularly exposed areas of the packaging material when this, by fold formation, is reformed into finished packages. In addition, Alifoil is relatively expensive to produce and is therefore a major contributory factor in greatly increasing the material and production costs of the laminated packaging material.
By way of alternative to the above-described Alifoil it is also known in the art to employ as gas barrier polymers of the barrier polymer type in paper or paperboard materials as described above. One among numerous known examples of such alternative barrier polymers possessing superior gas tightness properties is polyvinyl alcohol which, like the Alifoil, is applied on one side of the core layer between the core layer and the one outer liquid-tight plastic coating of the laminated packaging material.
Compared with an Alifoil, polyvinyl alcohol possesses several valuable properties which render polyvinyl alcohol particularly attractive as an alternative material to the Alifoil. First, it is considerably cheaper than an Alifoil, and further, a layer of polyvinyl alcohol is more ductile and extensible and therefore less crack-sensitive than the Alifoil. Polyvinyl alcohol possesses good gas tightness properties, in particular oxygen gas tightness properties, fully on a par with an Alifoil, at the same time as polyvinyl alcohol (as opposed to an Alifoil) is an approved and licensed material in food contexts and may, therefore, be used without legal hindrance in direct contact with foods.
One serious drawback inherent in polyvinyl alcohol is, however, that it is moisture-sensitive and rapidly loses its valuable gas-tightness properties if it is exposed to the effects of moisture.
The problem of polyvinyl alcohol""s moisture sensitivity is solved according to one prior art proposal in that the polyvinyl alcohol layer applied as the gas barrier is encapsulated or enveloped between surrounding liquid-tight plastic coatings which efficiently protect the interjacent polyvinyl alcohol layer against penetration by moisture and wet from both sides of the laminated packaging material. The application of such protective layers renders not only the overall structural build-up of the laminated packaging material more complex but also the method of producing the laminated packaging material, in that it requires at least one additional coating stage with associated coating equipment for applying these protective coatings.
According to another prior art method, the moisture sensitivity problems inherent in polyvinyl alcohol are solved by a chemical modification of the polyvinyl alcohol using chemical cross-linking agents with whose help the polyvinyl alcohol is chemically cross-linked for the formation of a cross-linked, water-resistant polyvinyl structure which withstands the action of moisture better than the pure polyvinyl alcohol without losing any gas barrier properties. Examples of such known cross-linking agents may be dialdehyde compounds (e.g. glyoxal) and acid anhydrides. Amongst other things, this prior art technology enjoys the advantage over and above the previously described encapsulation technique in that it makes for a simple production of the laminated packaging material without the need for additional application stages, at the same time as the thus produced packaging material displays a simpler material structure (fewer material layers) than the encapsulated material structure as described above. However, the problem inherent in this known chemically modified material structure is that the cross-linked polyvinyl alcohol layer is relatively rigid and unbending and therefore runs the risk, as in the known Alifoil structure, of cracking or rupturing in particularly exposed areas of the packaging material when this is reformed into finished packages.
One object of the present invention is therefore to obviate the above-described drawbacks in the prior art laminated packaging materials.
A further object of the present invention is to realise an improved laminated packaging material of the type described by way of introduction, without complicated material structure, as in the known packaging material employing an encapsulated polyvinyl alcohol layer.
Still a further object of the present invention is to realise a moisture and water-resistant polyvinyl alcohol material which may readily be reformed into liquid-tight, dimensionally stable packages possessing superior gas barrier properties, in particular superior oxygen gas barrier properties, without the risk of crack formation and other breaches in tightness in the cross-linked polyvinyl alcohol layer.
Solution
These and other objects and advantages will be attained according to the present invention by means of a laminated packaging material possessing the characterizing features as set forth in appended claim 1.
Expedient and practical embodiments of the laminated packaging material according to the invention have further been given the characterizing features as set forth in the claims.
Outline of the Invention
The combination of polyvinyl alcohol, chemical cross-linking agent and polysaccharide in the polyvinyl alcohol layer realises, according to the invention; a cross-linked polyvinyl alcohol structure which, through the chemical cross-linking, is water-resistant and which, at the same time as a result of the incorporation of the polysaccharide, is sufficiently flexible to be bendable without the risk of crack formation as a result of structural rigidity.
One preferred example of such a usable polysaccharide is chitosan, while examples of applicable chemical cross-linking agents may be conventional dialdehyde compounds, for example glyoxal and acid anhydrides.
The mixing ratio between polyvinyl alcohol and polysaccharide in the polyvinyl alcohol layer is between 99:1 and 50:50 on a weight basis, preferably, between 90:10 and 70:30.
The mixing ratio between the chemical cross-linking agent and the combination of polyvinyl alcohol and polysaccharide is between 99:1 and 50:50 on a weight basis.
The polyvinyl alcohol which is employed in the laminated packaging material according to the present invention should have a hydrolysis degree of at least 70 per cent, such as for example between 90 and 100 per cent.
In order to render the laminated packaging material according to the present invention readily convertible by fold forming and heat sealing into liquid-tight dimensionally stable packages, the packaging material is provided with outer, liquid-tight coatings of plastic, preferably thermoplastic, such as polyethylene, whereby the laminated packaging material may be sealed by conventional heat sealing techniques using existing equipment.
According to the present invention, the core layer may be a fibre-based material, such as paper or paperboard, or consist of plastic, such as polyolefin, polyester, polyamide etc.
According to a further aspect of the present invention, there will be realised a simple method of producing the laminated packaging material using existing production techniques and equipment. The method according to the present invention has the characterizing features as set forth in appended claim 13. Expedient and advantageous embodiments of the method according to the present invention have further been given the characterizing features as set forth in several of the claims.
According to yet a further aspect of the present invention, there will be realised a liquid-tight, dimensionally stable packaging container which has the characterizing features as claimed below.